In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. If, after imaging, the imaged regions of the imageable layer are removed revealing the underlying hydrophilic surface of the substrate, the precursor is positive working. Conversely, if the unimaged regions are removed, the precursor is negative working. In each instance, the regions of the imageable layer (i.e., the image areas) that remain are ink-receptive, and the revealed regions of the hydrophilic surface accept water and aqueous solutions, typically a fountain solution, and repel ink.
During imaging, scattered and reflected radiation can expose regions of the imageable layer in which exposure is not desired, reducing the sharpness of the image. Reflection of the imaging radiation by substrate or by the substrate/imaging layer interface can produce variations in the radiation intensity during imaging, producing non-uniform imaging. Radiation can scatter into regions of the imaging layer where imaging is not intended.
The use of antihalation dyes to prevent this unwanted exposure is well-known. The antihalation dye absorbs the imaging radiation. Because the scattered and reflected radiation has a longer path length in the imageable layer than radiation that impinges directly on the imageable layer, the scattered and reflected radiation is more strongly absorbed by the antihalation dye. Although the antihalation dye slightly reduces the overall speed of the imaging system, scattered and reflected radiation, which can produce a blurred image, is preferentially absorbed so that the overall definition and sharpness of image is increased.
The dye may be incorporated into a separate layer, typically between the imaging layer and the substrate, known as an antihalation layer. However, addition of this additional layer to the imageable element complicates the manufacturing process and makes it more expensive.
Alternatively, the dye may be incorporated into the imageable layer. However, the dye must be compatible with the other ingredients in the imageable layer, and except for absorbing some of the imaging radiation, not adversely affect with the physical and chemical changes that produce the image. When the image forming reaction is a free radical polymerization, the antihalation dye must not interfere with the free radical reaction.
Sypek, U.S. Pat. No. 5,543,262, the disclosure of which is incorporated herein by reference, discloses a composition that includes a photopolymerizable compound, a free radical generating component, and benzanthrone or a substituted benzanthrone. The benzanthrone or substituted benzanthrone substantially prevents photopolymerization when the composition is exposed to imaging radiation below a threshold amount but permits photopolymerization when the light sensitive composition is exposed to imaging radiation above a threshold amount. Although benzanthrone does not substantially affect imaging, in the manufacture of the imageable elements benzanthrone can sublime from the imageable composition during the drying step. Loss of benzanthrone produces an undesired variability in the finished product, and the sublimed benzanthrone contributes to nuisance dust in the manufacturing environment. Thus, a need exists for negative-working imageable elements that comprise an antihalation compound that does not substantially affect imaging, but that do not have these disadvantages.